A. Field of the Invention
The present invention relates to condenser/evaporator heat exchangers and to a method of using the same.
B. Brief Description of the Prior Art
Since the present invention is well adapted for use in conjunction with heat exchange systems where saline or brackish water is converted to potable water, the general state of the prior art with regard to heat exchangers will be given consideration with reference to such systems. A common arrangement for such systems is to employ two distinct heat exchangers. The first heat exchanger is generally a counterflow heat exchanger and is used to place the brine which is initially flowing into the system in heat exchange relationship with the potable water flowing from the system to transfer the heat from the potable water to the incoming brine and raise its temperature from ambient temperature to a higher temperature, possibly in the order of 200.degree. F. or so. The second heat exchanger is a condenser/evaporator type heat exchanger where the brine is pumped to the upper end of the heat exchanger and caused to fall as a thin film over one side of a set of heat exchange surfaces. At the same time, steam which is derived by heating the brine is compressed to a higher pressure and exposed to the opposite side of the heat exchange surfaces to be in heat exchange relationship with the brine film. This causes potable water to condense on the second set of surfaces and also causes evaporation of water from the brine flowing downwardly on the opposite side. This condensed water is collected and passed out through the first counterflow heat exchanger to raise the temperature of the incoming brine as described above.
In general, there have been two common arrangements for the elements which provide the heat exchange surfaces. One is to provide a plurality of plates arranged parallel to one another and spaced a short distance from each other, so that a plurality of adjacent passageways are formed by the various sets of plates; this is commonly called a flat plate heat exchanger. One heat exchange medium is directed through a first set of alternately spaced passages, while the second heat exchange medium is directed through the second set of passageways spaced intermittently with the first set. Thus, heat is transferred from one heat exchange medium to the other through the plates.
The second general arrangement for heat exchangers is to provide the heat exchange elements in the form of elongate tubes which extend through a heat exchange chamber and are spaced a moderate distance from one another. One heat exchange medium is directed into the interior of the tubes, while the other heat exchange medium is directed into the area between and around the outside of the tubes. In some instances, the second heat exchange medium flows in a direction transverse to the longitudinal axes of the tubes, and in other arrangements, the second heat exchange medium is directed parallel to the longitudinal axes of the tubes.
Since one of the main factors influencing the effectiveness of the heat exchanger is the heat transfer characteristics of the material separating the two heat exchange mediums, it has been quite common to fabricate the heat exchange elements from a metal which has a high thermal conductivity. However, for massive heat exchange installations, such as those used in producing potable water from saline water, the cost of providing and maintaining heat exchange elements in a quantity and size necessary to provide the heat exchange surface required, is a significant factor in determining whether the overall heat exchange system is economically feasible. This becomes particularly critical where metal is used as the material for the heat exchange elements, since the fabrication and installation of a vast number of metallic heat exchange elements can become a substantial portion of the cost of the entire system.
Accordingly, there have been attempts in the prior art to fabricate the heat exchange elements from other materials, and one of the results is research and development work in thin plastic film heat exchangers. Since plastic, in comparison to metal used in heat exchangers, is a relatively poor conductor of heat, for such films to operate with reasonable effectiveness, it is necessary to make the films quite thin to obtain adequate transfer of heat. The result is that the film material is generally relatively flexible and fragile in comparison to comparable metal heat exchange structures. When the thin film plastic is arranged as planar sheets to form the heat exchange surfaces (in the general configuration of metal panels), it becomes difficult to maintain the sheets in proper spaced relationship with respect to one another. One of the reasons for this is that to operate the heat exchanger, either as a counterflow heat exchanger or an evaporative type heat exchanger it is generally necessary to have at least some pressure differential between the two heat exchange mediums.
It has also been attempted in the prior art to provide thin film plastic heat exchangers in the form of tubular heat exchange elements. This alleviates to some extent the problem posed by pressure differential between the heat exchange mediums, since the higher pressure heat exchange medium can be directed into the interior of the tubes which are then caused to assume a generally circular configuration in response to the internal pressure. However, for practical commercial operation, these tubes must be provided in relatively long lengths, and there are quite often problems of instability in the tubes oscillating or becoming positioned against one another in response to the influence of the flow of the heat exchange medium or mediums either through or around the tubes. Not only does this create problems in preserving the structural integrity of the heat exchange structure, but it also creates a problem in the optimization of the heat transfer characteristics of the heat exchanger.
With regard to the various heat exchange devices shown in the literature of United States patents, the following are noted:
U.S. Pat. No. 1,955,261, Tryon et al, shows a heat exchanger where there are a plurality of tubes which are arranged in an alternating pattern and cast into a block made of a suitable metal, such as aluminum or copper.
U.S. Pat. No. 2,347,957, McCullough, shows a heat exchanger comprising a tubular member arranged in a circuitous pattern and having a number of fins extending therefrom to improve heat transfer.
U.S. Pat. No. 3,161,574, Elam, shows a condensor type heat transfer device where thin film plastic tubes are used as the heat exchange elements. Pressurized steam is directed into the interior of the tubes, and brine is directed as a film over the outside surface of the tubes.
U.S. Pat. No. 3,315,740, Withers, shows a heat exchanger made up of a tube bundle. The ends of the tubes are gathered together in a manner to form a fluid tight end portion of the tubular heat exchanger.
U.S. Pat. No. 3,493,040, Davison, shows a plate-type heat exchanger where the plates are formed with dimples to provide for proper spacing of the plates.
U.S. Pat. No. 3,537,935, Withers, shows a heat exchanger formed with plastic tubes, with one heat exchange medium being directed through the tubes and the other heat exchange medium being directed along a path transverse to the lengthwise axis of the tubes, commonly called a crossflow heat exchanger.
U.S. Pat. No. 3,616,835, Laurenty, is generally representative of a flat plate type heat exchanger.
U.S. Pat. No. 3,790,654, Bagley, teaches a method of extruding thin-walled honeycombed structure. While the teaching of this patent is not directed specifically toward heat exchangers, it does state that such honeycomb structures are used in reginerators, recuperators, radiators, catalyst carriers, filters, heat exchangers and the like.
U.S. Pat. No. 3,825,460, Yoshikawa et al, shows a carbonaceous honeycomb structure where tubular-like elements are formed into a variety of structures having elongate passageways, some of which are triangular, some of which are circular, and some of which are hexagonal.
U.S. Pat. No. 3,926,251, Pei, shows a counterflow heat exchanger where circular tubes are laid down, then expanded into contact with one another. In one embodiment, the tubes are arranged in a pattern so that the end passageways are formed as squares. In another configuration the tubes are arranged so that the end configuration of the passageways are hexagonal.
U.S. Pat. No. 3,948,317, Moore, discloses glass-ceramic tubes which are formed into a honeycomb configuration for use as heat exchangers.
U.S. Pat. No. 3,983,283, Bagley, discloses a ceramic honeycomb structure for use as a catalytic convertor or heat exchanger.
U.S. Pat. No. 4,002,040, Munters, shows a cross-current heat exchanger, where an airstream is cooled by evaporating moisture into a second air stream placed in heat exchange relationship with the first air stream.
U.S. Pat. No. 4,029,146, shows several configurations of a corrogated metal panel used as a heat exchanger.
The following patents are noted as broadly representative of various prior art devices: U.S. Pat. No. 2,820,744, Lighter; U.S. Pat. No. 3,168,450, Black; U.S. Pat. No. 3,239,000, Meagher; U.S. Pat. No. 3,367,843, Clive et al; U.S. Pat. No. 3,396,785, Kirsch; U.S. Pat. No. 3,428,529, Gumucio; U.S. Pat. No. 3,672,959, Sweet; U.S. Pat. No. 3,703,443, Evans; and U.S. Pat. No. 3,929,951, Shibata et al.